深入淺出Golang中的sync.Pool

type Pool struct {
    // GMP中，每一個P(協程排程器)會有一個陣列，陣列大小位localSize. 
 local     unsafe.Pointer 
 // p 陣列大小.
 localSize uintptr
 New func() any
}

// poolLocal 每個P(協程排程器)的本地pool.
type poolLocal struct {
 poolLocalInternal
    // 保證一個poolLocal佔用一個快取行
 pad [128 - unsafe.Sizeof(poolLocalInternal{})%128]byte
}

type poolLocalInternal struct {
 private any       // Can be used only by the respective P. 16
 shared  poolChain // Local P can pushHead/popHead; any P can popTail. 8
}

type poolChain struct {
 head *poolChainElt
 tail *poolChainElt
}

type poolChainElt struct {
 poolDequeue
 next, prev *poolChainElt
}

type poolDequeue struct {
 // head 高32位元，tail低32位元.
 headTail uint64
 vals []eface
}

// 儲存具體的value. 
type eface struct {
 typ, val unsafe.Pointer
}

func (p *Pool) pin() (*poolLocal, int) {
 // 返回執行當前協程的P(協程排程器),並且設定禁止搶佔.
 pid := runtime_procPin()
 s := runtime_LoadAcquintptr(&p.localSize) // load-acquire
 l := p.local                              // load-consume
 // pid < 核心數. 預設走該邏輯.
 if uintptr(pid) < s {
  return indexLocal(l, pid), pid
 }
 // 設定的P大於本機CPU核心數.
 return p.pinSlow()
}

// indexLocal 獲取當前P的poolLocal指標. 
func indexLocal(l unsafe.Pointer, i int) *poolLocal {
 // l p.local指標開始位置.
 // 我猜測這裡如果l為空，編譯階段會進行優化. 
 lp := unsafe.Pointer(uintptr(l) + uintptr(i)*unsafe.Sizeof(poolLocal{}))
 // uintptr真實的指標.
 // unsafe.Pointer Go對指標的封裝: 用於指標和結構體互相轉化.
 return (*poolLocal)(lp)
}

func (c *poolChain) pushHead(val any) {
 d := c.head
    // 初始化. 
 if d == nil {
  // Initialize the chain.
  const initSize = 8 // Must be a power of 2
  d = new(poolChainElt)
  d.vals = make([]eface, initSize)
  c.head = d
  // 將新構建的d賦值給tail.
  storePoolChainElt(&c.tail, d)
 }
 // 入隊.
 if d.pushHead(val) {
  return
 }
 // 佇列滿了.
 newSize := len(d.vals) * 2
 if newSize >= dequeueLimit {
        // 佇列大小預設為2的30次方. 
  newSize = dequeueLimit
 }

    // 賦值連結串列前後節點關係. 
 // prev.
 // d2.prev=d1.
 // d1.next=d2.
 d2 := &poolChainElt{prev: d}
 d2.vals = make([]eface, newSize)
 c.head = d2
 // next .
 storePoolChainElt(&d.next, d2)
 d2.pushHead(val)
}

// 入隊poolDequeue
func (d *poolDequeue) pushHead(val any) bool {
 ptrs := atomic.LoadUint64(&d.headTail)
 head, tail := d.unpack(ptrs)
 // head 表示當前有多少元素.
 if (tail+uint32(len(d.vals)))&(1<<dequeueBits-1) == head {
  return false
 }
 // 環型佇列. head&uint32(len(d.vals)-1) 表示當前元素落的位置一定在佇列上.
 slot := &d.vals[head&uint32(len(d.vals)-1)]

 typ := atomic.LoadPointer(&slot.typ)
 if typ != nil {
  return false
 }

 // The head slot is free, so we own it.
 if val == nil {
  val = dequeueNil(nil)
 }
    // 向slot寫入指標型別為*any，並且值為val.
 *(*any)(unsafe.Pointer(slot)) = val
    // headTail高32位元++
 atomic.AddUint64(&d.headTail, 1<<dequeueBits)
 return true
}